Items at the Event Horizon vs. colliding Black Holes

Items at the Event Horizon vs. colliding Black Holes

Items reaching a Black Hole's Event Horizon seem to be frozen in time from the point of view of an outside observer. On the other hand, Black Holes collide and merge, in finite steps, producing gravitational waves observable to outside observers. How is it possible these two facts not to contradict each other?

I don't know if this question is a good fit for the Stackexchange format, because it's a complex topic that would require a lot of discussion. But I'll try to provide an answer.

I think you're getting the wrong ideas from the image of the event horizon being frozen in time for the remote observer. While this is true, it also means the remote observer never actually gets any information from the event horizon proper.

If you're a remote observer, and you see any change that happens related to a black hole, by definition that change is not happening at the event horizon or inside it. It happens outside that region. The event horizon might as well not even exist from this p.o.v.

The merger of the two black holes is what the remote observer sees. But a hypothetical local observer, inside the event horizon, would have a different perception.

For the local observer, the whole history of anything outside the black hole takes place in an instant. The whole evolution of the universe, including the black hole colliding with another black hole, and the resulting object evaporating via Hawking radiation - all this stuff happens in an instant.

You really need to think of it this way: the remote and the local observer experience two completely separate realities, where the flow of time in one is completely disconnected from the flow of time in the other.

Behold! The Black Hole Collision Calculator!

Black holes have been the subject of intense interest ever since scientists began speculating about their existence. Originally proposed in the early 20th century as a consequence of Einstein’s Theory of General Relativity, black holes became a mainstream subject a few decades later. By 1971, the first physical evidence of black holes was found and by 2016, the existence of gravitational waves was confirmed for the first time.

This discovery touched off a new era in astrophysics, letting people know collision between massive objects (black holes and/or neutron stars) creates ripples in spacetime that can be detected light-years away. To give people a sense of how profound these events are, Álvaro Díez created the Black Hole Collision Calculator (BHCC) – a tool that lets you see what the outcome of a collision between a black hole and any astronomical object would be!

The BHCC is available at the Omni Calculator site, the same place where one can find calculators for determining how many alien civilizations could exist in our galaxy at any given time (the Alien Civilization Calculator), the time it would take to make an interstellar journey (the Space Travel Calculator), and the velocity needed to send a rocket into orbit (the Rocket Equation Calculator). And that’s just the Physics section!

Like the team that designed these calculators (Steven Wooding and Dominik Czernia), Diez is a physicist who parlayed his love of science into creating educational tools. After receiving his B.A. in fundamental Physics in his native Spain and completing two internships at CERN, he moved to Warsaw to pursue a master’s degree in computer modeling of physical phenomena.

During this time, Diez also worked for a series of Spanish blogs, writing about science and technology. Eventually, he would become inspired by the recent breakthroughs in astrophysics to create his own specialized calculator:

“In the last years, we’ve been learning exponentially more about black holes thanks to advancements in research like being able to detect gravitational waves. However, I’ve always found that these objects are surrounded by a mystery so I wanted to, ironically, shed some light into these obscure remnants of stars, but in a fun and interactive way. So I thought that an interactive calculator about black hole collisions could be a great way!”

“Mystery” is certainly an apt term when it comes to black holes. Put simply, these objects are the remnants of stars that have consumed the last of their fuel and undergone gravitational collapse. This causes them to shrink until they reach their Schwarzschild radius – which Diez also designed a calculator for! Also known as the “Event Horizon,” this is the boundary within which the laws of physics break down, resulting in a “singularity.”

The first-ever actual image of a black hole, taken in 2019 by the Event Horizon Telescope. Credit: EHT Collaboration

Because of the extreme gravitational forces involved, even light cannot escape the surface of a black hole, which is why they are completely inaccessible and undetectable (aka. “black”). The only way to study them is by observing their effect on the space around them, like the bright accretion disks that form beyond their Event Horizon (which is how the first image of a black hole was taken).

But as Diez explains on the BHCC home page, black holes are actually deceptively simple. “In general terms, black holes are one of the simplest objects to work with in the Universe,” he states. “The details are what make them strange and complicated.” Diez explained how the calculator works as follows (directions are also available on the site), showing how with a few approximations, things can be kept simple:

“So what the calculator does is a “first-approximation” calculation to determine the energy released when a black hole “eats” another astronomical object. The basic procedure involves calculating the initial and final potential gravitational energy of the object falling into the black hole. From there I just assume that the black hole is a non-rotating one (for simplicity) and apply a known efficiency factor to know how much of that energy is released. The results truly show the magnitude of black holes and the energies they are capable of releasing during a collision.”

As a thought experiment, Diez offers a calculation of what it would look like if a black hole collided with and consumed a Sun-like star. Basically, if a main sequence G-type (yellow dwarf) star were to collide and be consumed by a black hole of 5 solar masses, it would produce 107.257 x 10 15 Megajoules of energy waves, which is about 27.9 x 10 24 times what we consume here on Earth annually.

In terms of its intended purpose (making science fun vs. academic applications), Diez indicated that the calculator falls somewhere in between. In the end, his real goal was to make astrophysics accessible:

“I want it to be a stepping stone for people interested in science that haven’t had any formal training in astronomy or cosmology. The idea of this calculator is not to get research-level results but to make research results understandable for everyone. I’ve always loved doing scientific research but for me, nothing beats helping others see how amazing our universe is.”

In addition to the Black Hole Calculator and the Schwarzschild Radius Calculator, Diez also developed the Black Hole Temperature Calculator, which allows users to gauge the temperature of a black hole based on its mass. He also contributed an Exoplanet Discovery Calculator, which allows users to infer the mass and orbit of an exoplanet based on the star and the detection method involved.

He’s also the creator of the Hubble Law Distance Calculator and the Universe Expansion Calculator, which gives users the ability to calculate the speed any galaxy is moving at relative to us and cosmic expansion based on Hubble’s Law. He also partnered with Dr. Milosz Panfil, who holds a Ph.D. in quantum physics (also from the University of Warsaw), to create the Orbital Period Calculator – to determine the orbits of planets and binary stars.

An illustration of cosmic expansion. Credit: NASA’s Goddard Space Flight Center Conceptual Image Lab

What’s next, you might ask? Well, it turns out Diez has some thoughts on that as well. Mostly, they involve creating more calculators, which he feels are an underutilized resource when it comes to science communication. As he said, he’s also hoping to expand beyond the realm of physics:

“I’ve been working on some Coronavirus calculators to help people make better decisions in these difficult times, and I’m also working on a collaboration with universities all over Europe to include calculators as a way to make Researchers’ Night (the biggest science communication event I know of) be more interactive and appealing for everyone. Science works, and a scientifically-minded society works best!”

One of the greatest assets of the information age is the way it’s allowing educators and scientists to communicate directly with a wider audience. Through shared tools and apps, people are able to see for themselves just how some of the greatest scientific breakthroughs and concepts in history work. Like modern-day space exploration, accessibility is the key!

To learn more about the many, MANY, tools and apps they have, head on over to the Omni Calculator site and have a look around.

Addendum: The value for the Black Hole Mass (before Collision) is a set value. You can adjust it, but it will not affect the outcome. The author has indicated that this is by design for the sake of keeping this introductory calculator simple and the absence of relativistic effects is apparently for the same purpose. He further states that the energy approximation of 7% (of the object’s mass) is consistent with similar approximations made in a 2014 NASA study.

This was in response to criticism/concerns raised by Forbes senior science contributor Ethan Siegel.

In a first, astronomers watch a black hole’s corona disappear, then reappear

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It seems the universe has an odd sense of humor. While a crown-encrusted virus has run roughshod over the world, another entirely different corona about 100 million light years from Earth has mysteriously disappeared.

For the first time, astronomers at MIT and elsewhere have watched as a supermassive black hole’s own corona, the ultrabright, billion-degree ring of high-energy particles that encircles a black hole’s event horizon, was abruptly destroyed.

The cause of this dramatic transformation is unclear, though the researchers guess that the source of the calamity may have been a star caught in the black hole’s gravitational pull. Like a pebble tossed into a gearbox, the star may have ricocheted through the black hole’s disk of swirling material, causing everything in the vicinity, including the corona’s high-energy particles, to suddenly plummet into the black hole.

The result, as the astronomers observed, was a precipitous and surprising drop in the black hole’s brightness, by a factor of 10,000, in under just one year.

“We expect that luminosity changes this big should vary on timescales of many thousands to millions of years,” says Erin Kara, assistant professor of physics at MIT. “But in this object, we saw it change by 10,000 over a year, and it even changed by a factor of 100 in eight hours, which is just totally unheard of and really mind-boggling.”

Following the corona’s disappearance, astronomers continued to watch as the black hole began to slowly pull together material from its outer edges to reform its swirling accretion disk, which in turn began to spin up high-energy X-rays close to the black hole’s event horizon. In this way, in just a few months, the black hole was able to generate a new corona, almost back to its original luminosity.

“This seems to be the first time we’ve ever seen a corona first of all disappear, but then also rebuild itself, and we’re watching this in real-time,” Kara says. “This will be really important to understanding how a black hole’s corona is heated and powered in the first place.”

Kara and her co-authors, including lead author Claudio Ricci of Universidad Diego Portales in Santiago, Chile, have published their findings today in Astrophysical Journal Letters. Co-authors from MIT include Ron Remillard, and Dheeraj Pasham.

A nimble washing machine

In March 2018, an unexpected burst lit up the view of ASSASN, the All-Sky Automated Survey for Super-Novae, that surveys the entire night sky for supernova activity. The survey recorded a flash from 1ES 1927+654, an active galactic nucleus, or AGN, that is a type of supermassive black hole with higher-than-normal brightness at the center of a galaxy. ASSASN observed that the object’s brightness jumped to about 40 times its normal luminosity.

“This was an AGN that we sort of knew about, but it wasn’t very special,” Kara says. “Then they noticed that this run-of-the-mill AGN became suddenly bright, which got our attention, and we started pointing lots of other telescopes in lots of other wavelengths to look at it.”

The team used multiple telescopes to observe the black hole in the X-ray, optical, and ultraviolet wave bands. Most of these telescopes were pointed at the the black hole periodically, for example recording observations for an entire day, every six months. The team also watched the black hole daily with NASA’s NICER, a much smaller X-ray telescope, that is installed aboard the International Space Station, with detectors developed and built by researchers at MIT.

“NICER is great because it’s so nimble,” Kara says. “It’s this little washing machine bouncing around the ISS, and it can collect a ton of X-ray photons. Every day, NICER could take a quick little look at this AGN, then go off and do something else.”

With frequent observations, the researchers were able to catch the black hole as it precipitously dropped in brightness, in virtually all the wave bands they measured, and especially in the high-energy X-ray band — an observation that signaled that the black hole’s corona had completely and suddenly vaporized.

“After ASSASN saw it go through this huge crazy outburst, we watched as the corona disappeared,” Kara recalls. “It became undetectable, which we have never seen before.”

A jolting flash

Physicists are unsure exactly what causes a corona to form, but they believe it has something to do with the configuration of magnetic field lines that run through a black hole’s accretion disk. At the outer regions of a black hole’s swirling disk of material, magnetic field lines are more or less in a straightforward configuration. Closer in, and especially near the event horizon, material circles with more energy, in a way that may cause magnetic field lines to twist and break, then reconnect. This tangle of magnetic energy could spin up particles swirling close to the black hole, to the level of high-energy X-rays, forming the crown-like corona that encircles the black hole.

Kara and her colleagues believe that if a wayward star was indeed the culprit in the corona’s disappearance, it would have first been shredded apart by the black hole’s gravitational pull, scattering stellar debris across the accretion disk. This may have caused the temporary flash in brightness that ASSASN captured. This “tidal disruption,” as astronomers call such a jolting event, would have triggered much of the material in the disk to suddenly fall into the black hole. It also might have thrown the disk’s magnetic field lines out of whack in a way that it could no longer generate and support a high-energy corona.

This last point is a potentially important one for understanding how coronas first form. Depending on the mass of a black hole, there is a certain radius within which a star will most certainly be pulled in by a black hole’s gravity.

“What that tells us is that, if all the action is happening within that tidal disruption radius, that means the magnetic field configuration that’s supporting the corona must be within that radius,” Kara says. “Which means that, for any normal corona, the magnetic fields within that radius are what’s responsible for creating a corona.”

The researchers calculated that if a star indeed was the cause of the black hole’s missing corona, and if a corona were to form in a supermassive black hole of similar size, it would do so within a radius of about 4 light minutes — a distance that roughly translates to about 75 million kilometers from the black hole’s center.

“With the caveat that this event happened from a stellar tidal disruption, this would be some of the strictest constraints we have on where the corona must exist,” Kara says.

The corona has since reformed, lighting up in high-energy X-rays which the team was also able to observe. It’s not as bright as it once was, but the researchers are continuing to monitor it, though less frequently, to see what more this system has in store.

“We want to keep an eye on it,” Kara says. “It’s still in this unusual high-flux state, and maybe it’ll do something crazy again, so we don’t want to miss that.”

When testing Einstein's theory of general relativity, small modeling errors add up fast

Small modeling errors may accumulate faster than previously expected when physicists combine multiple gravitational wave events (such as colliding black holes) to test Albert Einstein's theory of general relativity, suggest researchers at the University of Birmingham in the United Kingdom. The findings, published June 16 in the journal iScience, suggest that catalogs with as few as 10 to 30 events with a signal-to-background noise ratio of 20 (which is typical for events used in this type of test) could provide misleading deviations from general relativity, erroneously pointing to new physics where none exists. Because this is close to the size of current catalogs used to assess Einstein's theory, the authors conclude that physicists should proceed with caution when performing such experiments.

"Testing general relativity with catalogs of gravitational wave events is a very new area of research," says Christopher J. Moore, a lecturer at the School of Physics and Astronomy & Institute for Gravitational Wave Astronomy at the University of Birmingham in the United Kingdom and the lead author of the study. "This is one of the first studies to look in detail at the importance of theoretical model errors in this new type of test. While it is well known that errors in theoretical models need to be treated carefully when you are trying to test a theory, we were surprised by how quickly small model errors can accumulate when you start combining events together in catalogs."

In 1916, Einstein published his theory of general relativity, which explains how massive celestial objects warp the interconnected fabric of space and time, resulting in gravity. The theory predicts that violent outer space incidents such as black hole collisions disrupt space-time so severely that they produce ripples called gravitational waves, which zoom through space at the speed of light. Instruments such as LIGO and Virgo have now detected gravitational wave signals from dozens of merging black holes, which researchers have been using to put Einstein's theory to the test. So far, it has always passed. To push the theory even further, physicists are now testing it on catalogs of multiple grouped gravitational wave events.

"When I got interested in gravitational wave research, one of the main attractions was the possibility to do new and more stringent tests of general relativity," says Riccardo Buscicchio, a PhD student at the School of Physics and Astronomy & Institute for Gravitational Wave Astronomy and a co-author of the study. "The theory is fantastic and has already passed a hugely impressive array of other tests. But we know from other areas of physics that it can't be completely correct. Trying to find exactly where it fails is one of the most important questions in physics."

However, while larger gravitational wave catalogs could bring scientists closer to the answer in the near future, they also amplify the potential for errors. Since waveform models inevitably involve some approximations, simplifications, and modeling errors, models with a high degree of accuracy for individual events could prove misleading when applied to large catalogs.

To determine how waveform errors grow as catalog size increases, Moore and colleagues used simplified, linearized mock catalogs to perform large numbers of test calculations, which involved drawing signal-to-noise ratios, mismatch, and model error alignment angles for each gravitational wave event. The researchers found that the rate at which modeling errors accumulate depends on whether or not modeling errors tend to average out across many different catalog events, whether deviations have the same value for each event, and the distribution of waveform modeling errors across events.

"The next step will be for us to find ways to target these specific cases using more realistic but also more computationally expensive models," says Moore. "If we are ever to have confidence in the results of such tests, we must first have as a good an understanding as possible of the errors in our models."

This work was supported by a European Union H2020 ERC Starting Grant, the Leverhulme Trust, and the Royal Society.

Astronomers observe collision of 2 black holes — 7 billion years later

2:47 UBC researchers detect massive black hole collision
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Black holes are getting stranger — even to astronomers. They’ve now detected the signal from a long ago violent collision of two black holes that created a new one of a size that had never been seen before.

“It’s the biggest bang since the Big Bang observed by humanity,” said Caltech physicist Alan Weinstein, who was part of the discovery team.

Researchers with the University of British Columbia were also on that team.

Black holes are compact regions of space so densely packed that not even light can escape. Until now, astronomers only had observed them in two general sizes. There are “small” ones called stellar black holes that are formed when a star collapses and are about the size of small cities. And there are supermassive black holes that are millions, maybe billions, of times more massive than our sun and around which entire galaxies revolve.

According to astronomers’ calculations, anything in between didn’t quite make sense, because stars that grew too big before collapse would essentially consume themselves, leaving no black holes.

Star collapses couldn’t create stellar black holes much bigger than 70 times the mass of our sun, scientists thought, according to physicist Nelson Christensen, research director of the French National Centre for Scientific Research.

Then in May 2019 two detectors picked up a signal that turned out to be the energy from two stellar black holes — each large for a stellar black hole — crashing into each other. One was 66 times the mass of our sun and the other a husky 85 times the mass of the sun.

The end result: The first ever discovered intermediate black hole, at 142 times the mass of the sun.

5:51 Black hole expert explains new discovery on GNM

Lost in the collision was an enormous amount of energy in the form of a gravitational wave, a ripple in space that travels at the speed of light. It was that wave that physicists in the United States and Europe, using detectors called LIGO and Virgo, captured last year. After deciphering the signal and checking their work, scientists published the results Wednesday in Physical Review Letters and Astrophysical Journal Letters.

How Do Black Holes Work?

Black holes are possibly the most mysterious objects in the universe, and their name doesn’t really help their reputation. Of course, it’s also a little misleading. Black holes aren’t really holes at all, but instead they’re areas of incredibly strong gravity resulting from (we think) a singularity. This still doesn’t answer the fundamental question: how do black holes work? Here’s what we think we know:

1. First, the black hole is formed.
Black holes don’t just appear. They have to be created by cosmic processes just like everything else. They are most commonly formed when a massive star (far more massive than our own sun) dies and collapses in on itself. All that mass crams together, becoming denser and denser.

2. When mass is squeezed into an infinite density, you get a singularity.
A singularity is, in essence, a point in space with infinite density. It may seem ridiculous to say that anything in reality is infinite, but in physics, not everything makes sense. Black holes exist, therefore the properties we have predicted should exist in some form.

3. Strong enough gravity can stop anything from escaping, including light.
So a black hole is really a singularity. Where does the black part come in? A singularity has infinite density, which translates to an incredibly strong gravitational field. Anything nearby is pulled in by the force of gravity, just like on Earth. However, unlike on Earth, that gravity is so strong that it can stop light from escaping. That’s why the area is black there’s no light escaping from it, so we can’t see it. We can only see what is around it.

Edwin Carter



I'm having trouble reconciling these three things I've heard about black holes:

1) If you fall into a sufficiently large black hole, you won't experience anything in particular when crossing the event horizon. You'll have some time to experience being inside the black hole until tidal forces eventually grow dangerous.
2) Someone observing you falling into the black hole from the outside will never actually see you fall in, but rather observe your clock slowing down more and more, as all signs of you will grow weaker and longer in wavelength in an asymptotic approach to the event horizon.
3) All black holes, even the largest, eventually "evaporate", even if that won't fully happen for

My amateur interpretation of the first two items is that as you fall into the black hole, in your own frame of reference, the universe you leave behind will rapidly age from your perspective. Once you cross the event horizon, all you once knew will essentially be infinitely in the past, and you will then be in what could be considered the infinite future to the rest of the universe.

If I consider the third item, however, it seems that this infinite future can't exist, at least not inside the black hole. The black hole falls apart before then.

This leads me to guess that you might not ever be able to experience the inside of a black hole after all. Rather, you'd simply disintegrate as you cross the event horizon, scattering your mass across the distant future.

What if you fell into a black hole?

Here's where it starts to get bad. Tidal forces -- so named because similar gravitational forces between the moon and the Earth cause ocean tides -- increase dramatically as the distance between you and the black hole shrinks. This means the gravity acting on your feet is much stronger than the gravity acting on your head. As a result, your feet begin to accelerate much faster than your head. Your body stretches out, not uncomfortably at first, but over time, the stretching will become more severe. Astronomers call this spaghettification because the intense gravitational field pulls you into a long, thin piece of spaghetti.

When you start feeling pain depends on the size of the black hole. If you're falling into a supermassive black hole, you'll begin to notice the tidal forces within about 600,000 kilometers (372,822 miles) of the center -- after you've already crossed the event horizon [source: Bunn]. If you're falling into a stellar black hole, you'll start feeling uncomfortable within 6,000 kilometers (3,728 miles) of the center, long before you cross the horizon [source: Bunn].

Either way, spaghettification leads to a painful conclusion. When the tidal forces exceed the elastic limits of your body, you'll snap apart at the weakest point, probably just above the hips. You'll see your lower half floating next to you, and you'll see it begin to stretch anew as tidal forces latch onto it. The same thing happens to your torso, of course, until each half snaps a second time. In a matter of seconds, you're a goner, reduced to a string of disconnected atoms that march into the black hole's singularity like ants disappearing into a colony.

It's not a great way to die, but there's one consolation: In space, no one can hear you scream.

An international team discovers the "heaviest black hole collision" might be a boson star merger

The hypothetical stars are among the simplest exotic compact objects proposed and constitute well founded dark matter candidates. Within this interpretation, the team is able to estimate the mass of a new particle constituent of these stars, an ultra-light boson with a mass billions of times smaller than that of the electron. Their analysis has been published in the journal Physical Review Letters on 24 February 2021.

The team is co-led by Dr. Juan Calderón Bustillo, a former professor from the Department of Physics at CUHK and now "La Caixa Junior Leader - Marie Curie Fellow", at the Galician Institute of High Energy Physics, and Dr. Nicolás Sanchis-Gual, a postdoctoral researcher at the University of Aveiro and at the Instituto Superior Técnico (University of Lisbon). Other collaborators came from the University of Valencia, the University of Aveiro and Monash University. Samson Hin Wai Leong, a second-year undergraduate at CUHK, also participated.

Gravitational waves are ripples in the fabric of spacetime that travel at the speed of light. Predicted in Einstein's General Theory of Relativity, they originate in the most violent events of the Universe, carrying information about their sources. Since 2015, the advanced detectors of the Laser Interferometer Gravitational Wave Observatory (LIGO) and Virgo have observed around 50 gravitational wave signals originated in the coalescence and merger of two of the most mysterious entities in the Universe -- black holes and neutron stars.

In September 2020, LVC, the joint body of the LIGO Scientific Collaboration and the Virgo Collaboration, announced the detection of the gravitational wave signal GW190521. According to the LVC analysis, in which the CUHK group led by Professor Tjonnie Li, Associate Professor of the Department of Physics at CUHK was deeply involved, the signal was consistent with the collision of two black holes of 85 and 66 times the mass of the Sun, which produced a final 142 solar mass black hole. The latter was the first member ever found of a new black hole family -- intermediate-mass black holes. According to Professor Tjonnie Li, this discovery was of paramount importance because such black holes had been long considered the missing link between the stellar-mass black holes that form from the collapse of stars, and the supermassive black holes that hide in the centre of almost every galaxy.

Despite its significance, the observation of GW190521 poses an enormous challenge to the current understanding of stellar evolution, because one of the black holes merged has a "forbidden" size. The alternative explanation proposed by the team brings a new direction for the study. Dr. Nicolás Sanchis-Gual explained, "Boson stars are objects almost as compact as black holes but, unlike them, they do not have a 'no return' surface or event horizon. When they collide, they form a boson star that can become unstable, eventually collapsing to a black hole, and producing a signal consistent with what LVC observed last year. Unlike regular stars, which are made of what we commonly know as matter, boson stars are made up of ultra-light bosons. These bosons are one of the most appealing candidates for constituting dark matter forming around 27% of the Universe."

The team compared the GW190521 signal to computer simulations of boson star mergers and found that these actually explain the data slightly better than the analysis conducted by LVC. The result implies that the source would have different properties than stated earlier. Dr. Juan Calderón Bustillo said, "First, we would not be talking about colliding black holes anymore, which eliminates the issue of dealing with a forbidden black hole. Second, because boson star mergers are much weaker, we infer a much closer distance than the one estimated by LVC. This leads to a much larger mass for the final black hole, of about 250 solar masses, so the fact that we have witnessed the formation of an intermediate-mass black hole remains true."

Professor Toni Font, from the University of Valencia and one of the co-authors, explained that even though the analysis tends to favour "by design" the merging black holes hypothesis, a boson star merger is actually slightly preferred by the data, although in a non-conclusive way. Despite the computational framework of the current boson star simulations being still fairly limited and subject to major improvements, the team will further develop a more evolved model and study similar gravitational wave observations under the boson star merger assumption.

According to another co-author, Professor Carlos Herdeiro from the University of Aveiro, the finding not only involves the first observation of boson stars, but also that of their building block, a new particle known as the ultra-light boson. Such ultra-light bosons have been proposed as the constituents of what we know as dark matter. Moreover, the team can actually measure the mass of this putative new dark matter particle and a value of zero is discarded with high confidence. If it is confirmed by the subsequent analysis of GW190521 and other gravitational wave observations, the result would provide the first observational evidence for a long sought dark matter candidate.

Samson Hin Wai Leong, a student who joined the summer undergraduate research internship programme of CUHK added, "I worked with Professor Calderón Bustillo on the design of the software of this project, which successfully speeded up the calculations of the study, and eventually we were able to release our results immediately after LVC published their analysis. It is thrilling to work at the frontier of physics with the multicultural team and think about seeking a 'darker' origin of the ripples in spacetime, at the same time proving the existence of a dark matter particle."

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.

Are black holes made of dark energy?

Objects like Powehi, the recently imaged supermassive compact object at the center of galaxy M87, might actually be GEODEs. The Powehi GEODE, shown to scale, would be approximately 2/3 the radius of the dark region imaged by the Event Horizon Telescope. This is nearly the same size expected for a black hole. The region containing Dark Energy (green) is slightly larger than a black hole of the same mass. The properties of any crust (purple), if present, depend on the particular GEODE model. Credit: EHT collaboration NASA/CXC/Villanova University

Two University of Hawaii at Manoa researchers have identified and corrected a subtle error that was made when applying Einstein's equations to model the growth of the universe.

Physicists usually assume that a cosmologically large system, such as the universe, is insensitive to details of the small systems contained within it. Kevin Croker, a postdoctoral research fellow in the Department of Physics and Astronomy, and Joel Weiner, a faculty member in the Department of Mathematics, have shown that this assumption can fail for the compact objects that remain after the collapse and explosion of very large stars.

"For 80 years, we've generally operated under the assumption that the universe, in broad strokes, was not affected by the particular details of any small region," said Croker. "It is now clear that general relativity can observably connect collapsed stars—regions the size of Honolulu—to the behavior of the universe as a whole, over a thousand billion billion times larger."

Croker and Weiner demonstrated that the growth rate of the universe can become sensitive to the averaged contribution of such compact objects. Likewise, the objects themselves can become linked to the growth of the universe, gaining or losing energy depending on the objects' compositions. This result is significant since it reveals unexpected connections between cosmological and compact object physics, which in turn leads to many new observational predictions.

One consequence of this study is that the growth rate of the universe provides information about what happens to stars at the end of their lives. Astronomers typically assume that large stars form black holes when they die, but this is not the only possible outcome. In 1966, Erast Gliner, a young physicist at the Ioffe Physico-Technical Institute in Leningrad, proposed an alternative hypothesis that very large stars should collapse into what could now be called Generic Objects of Dark Energy (GEODEs). These appear to be black holes when viewed from the outside but, unlike black holes, they contain Dark Energy instead of a singularity.

In 1998, two independent teams of astronomers discovered that the expansion of the Universe is accelerating, consistent with the presence of a uniform contribution of Dark Energy. It was not recognized, however, that GEODEs could contribute in this way. With the corrected formalism, Croker and Weiner showed that if a fraction of the oldest stars collapsed into GEODEs, instead of black holes, their averaged contribution today would naturally produce the required uniform Dark Energy.

The results of this study also apply to the colliding double star systems observable through gravitational waves by the LIGO-Virgo collaboration. In 2016, LIGO announced the first observation of what appeared to be a colliding double black hole system. Such systems were expected to exist, but the pair of objects was unexpectedly heavy—roughly 5 times larger than the black hole masses predicted in computer simulations. Using the corrected formalism, Croker and Weiner considered whether LIGO-Virgo is observing double GEODE collisions, instead of double black hole collisions. They found that GEODEs grow together with the universe during the time leading up to such collisions. When the collisions occur, the resulting GEODE masses become 4 to 8 times larger, in rough agreement with the LIGO-Virgo observations.

Croker and Weiner were careful to separate their theoretical result from observational support of a GEODE scenario, emphasizing that "black holes certainly aren't dead. What we have shown is that if GEODEs do exist, then they can easily give rise to observed phenomena that presently lack convincing explanations. We anticipate numerous other observational consequences of a GEODE scenario, including many ways to exclude it. We've barely begun to scratch the surface."

The study, Implications of Symmetry and Pressure in Friedmann Cosmology: I. Formalism, is published in the August 28, 2019 issue of The Astrophysical Journal and is available online.

PG 1302-102: The Final Stages Before a Merger?

As mentioned earlier, black hole mergers are complicated and often require computers to help us. Wouldn&apost it be great if we had something to compare to theory? Enter PG 1302-102, a quasar which is exhibiting a weird repeating light signal which seems to match what we would see for the final steps of a black hole merger where the two objects get ready to meld. They may even be 1 millionth of a light year apart, based on archival data showing that indeed the roughly 5-year light cycle is present. It would appear to be a black hole pair about 0.02 to 0.06 light years apart and moving at about 7-10% the speed of light, with the light being periodic because of the constant tugging of the black holes. Amazingly, they move so fast that relativistic effects on space-time pull the light away from us and cause a dimming effect, with an opposite effect occurring when moving towards us. This in conjunction with the Doppler effect results in the pattern we see. However, it is possible that the light readings could come from an erratic accretion disc, but data from Hubble and GALEX in several different wavelengths over 2 decades points to the binary black hole picture. Additional data was found using the Catalina Real-time Transient Survey (active since 2009 and making use of 3 telescopes).The Survey hunted 500 million objects over a span of 80% of the sky. The activity of that region can be measured as an output of brightness, and 1302 displayed a pattern that models indicate would arise from two black holes falling into each other. 1302 had the best data, showing a variation with corresponded with a period of 60 months. Scientists did have to make that the changes in brightness were not caused by a single black hole’s accretion disc and the precession of the jet lined up in an optimal way. Fortunately, the period for such an event is 1,000 – 1,000,000 years, so it wasn’t difficult to rule out. Out of 247,000 quasars that were seen during the study, 20 more may have a pattern similar to 1302 such as PSO J334.2028+01.4075 (California, Rzetelny 24 Sept. 2015, Maryland, Betz, Rzetelny 08 Jan. 2015, Carlisle, JPL "Funky").